Recently developed ion sources that produce ions from photoionized gases (collectively referred to herein as photoionized gas ion source—PIGIS), have the potential to offer superior brightness and reduced energy spread ion beams, when compared with alternative ion sources. These charged particle sources could find useful application in improving the performance of focused ion beam instrumentation in particular. A PIGIS is characterized by the use of photoionization of gaseous atoms to produce a source of charged particles and incorporates a photoionization subsystem having one or more beams of laser radiation that ionize gaseous atoms to form pairs of ion and electrons. Beams of both ions and electrons can be created by applying a constant electric field to the region in space in which the charged particles are created. The photoionization subsystem and method for configuring and applying such subsystem are herein referred to as the photoionization process.
The photoionization process is critical to the performance of a PIGIS. The utility of the PIGIS for focused ion beam applications can be enhanced if the photoionization process:                1) achieves a high ionization efficiency;        2) creates ions that may be formed into a beam with low chromatic energy spread;        3) creates ions that have a variance in their transverse velocities (relative to the beam's axis) that are substantially similar to that of the collection of cold neutral atoms from which the ions are created; and        4) suppresses the creation of more than one ion in close proximity to another.        
Unfortunately, conventional photoionization systems are not designed to adequately meet the above criteria. Conventional photoionization systems may employ lasers with a photon energy substantially equal to the electric field-free ionization potential (FFIP) and are not configured to excite resonant structure in the photoionization spectrum. Conventional PIGISes are described in the references Knuffman, et al, titled “Nanoscale focused ion beam from laser-cooled lithium atoms” New J. Phys. 13 103035 (2011) and Reijnders, et al, titled “Low-Energy-Spread Ion Bunches from a Trapped Atomic Gas Phys. Rev. Lett. 102 034802 (2009).